What is heresy?

I served as a lecturer 15 years ago. I talked about how to evangelize to heresy members. One of foreigners came to me during an intercession and talked to me in Japanese, “Excuse me, but may I have a moment with you?” He asked me whether Catholic is a heresy or not. To tell the truth, this man who is older than me was a priest.

My answer was like these below.
The Bible is the only criteria whether it is a heresy or not.
It is not correct to misuse and stretch the definition of heresy.
To have a different interpretation from a group where he or she belongs to or a different theology what he or she adheres to is not a heresy.
In the Bible, it says if they teach different Jesus, different gospel, for example not only faith but also action is needed to be saved, and denial of the truth, for example the Bible was given by the Spirit of God, they are heresy.

The priest was relieved to hear what I had said and confessed some of Catholic’s doctrines are unbiblical and there is a movement inside of Catholic group to let them bring back to the Bible.

In the Bible, Christians shouldn’t call themselves “~ denomination”, “~ ism”, Protestant or Catholic. The people who repent their sin, believe Jesus, and are born again are the only Christians in the Bible.

Act 11:25~26